Collective properties of nucleus-nucleus collisions from AGS to LHC energies

TL;DR

This paper investigates azimuthal anisotropies in heavy-ion collisions across a wide energy range using the PHSD model, highlighting the increasing role of quark-gluon degrees of freedom in the early collision stages.

Contribution

It demonstrates that the PHSD transport approach successfully describes the energy dependence of flow harmonics, emphasizing the significance of partonic effects at higher energies.

Findings

Elliptic flow $v_2$ increases with collision energy.

Higher-order harmonics $v_3$ and $v_4$ show growing deviations between models.

The ratio $v_4/(v_2)^2$ varies with energy, centrality, and transverse momentum.

Abstract

The azimuthal anisotropies of the collective transverse flow of charged hadrons are investigated in a wide range of heavy-ion collision energies within the microscopic Parton-Hadron-String Dynamics (PHSD) transport approach which incorporates explicit partonic degrees-of-freedom in terms of strongly interacting quasiparticles (quarks and gluons) in line with an equation-of-state from lattice QCD as well as the dynamical hadronization and hadronic collision dynamics in the final reaction phase. The experimentally observed increase of the elliptic flow $v_2$ of charged hadrons with collision energy is successfully described in terms of the PHSD approach. The analysis of higher-order harmonics $v_3$ and $v_4$ in the azimuthal angular distribution shows a similar tendency of growing deviations between partonic and purely hadronic models with increasing collision energy. This demonstrates that the excitation functions of azimuthal anisotropies reflect the increasing role of quark-gluon degrees of freedom in the early phase of relativistic heavy-ion collisions. Furthermore, the specific variation of the ratio $v_4/(v_2)^2$ with respect to bombarding energy, centrality and transverse momentum is found to provide valuable information on the underlying partonic dynamics.